System and method for cell switching

ABSTRACT

A method for operating a first distributed unit (DU) includes participating in a link layer context transfer with a second DU, establishing a user data path for a mobile device, the user data path connecting the mobile device with a centralized unit (CU) including a radio control protocol entity communicating with the mobile device, and storing first user data for the mobile device received from the CU and second user data for the mobile device received from the second DU. The method includes establishing a connection with the mobile device, sending the first user data and the second user data to the mobile device, and adapting the link layer context to operate in the first DU for exchanging data between the CU and the mobile device, wherein adapting the link layer context includes associating an uppermost protocol layer of the first DU with a lowest protocol layer of the CU.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/478,908, filed on Apr. 4, 2017, entitled “System and Method for CellSwitching,” which claims the benefit of U.S. Provisional Application No.62/348,475, filed on Jun. 10, 2016, entitled “System and Method for CellSwitching,” all of which applications are hereby incorporated herein byreference.

TECHNICAL FIELD

The present invention relates generally to a system and method fordigital communications, and, in particular embodiments, to a system andmethod for cell switching.

BACKGROUND

The new radio (NR) design for Fifth Generation (5G) cellular radioaccess technology (RAT) is widely assumed to divide radio networkfunctionality between entities referred to as centralized units (orcontrol units) (CU) and distributed units (DU). The division of theradio protocol stack between these entities may be flexible, but atleast it is expected that the upper layers, e.g., the user planecomprising the packet data convergence protocol (PDCP) sublayer and thecontrol plane comprising the PDCP+radio resource control (RRC) sublayerswould be in the CU, while the physical (PHY) sublayer would be in theDU. The intervening Layer 2 sublayers may be in either the CU or the DU.However, it is likely that the media access control (MAC) sublayer wouldalso be located in the DU to avoid backhaul latency in the schedulingprocess.

SUMMARY

Example embodiments provide a system and method for cell switching.

In accordance with an example embodiment, a method for operating a firstdistributed unit (DU) is provided. The method includes participating, bythe first DU, in a link layer context transfer with a second DU,establishing, by the first DU, a user data path for a mobile device, theuser data path connecting the mobile device with a centralized unit (CU)including a radio control protocol entity communicating with the mobiledevice, storing, by the first DU, first user data for the mobile devicereceived from the CU, storing, by the first DU, second user data for themobile device received from the second DU, establishing, by the firstDU, a connection with the mobile device, sending, by the first DU, thefirst user data and the second user data to the mobile device, andadapting, by the first DU, the link layer context to operate in thefirst DU for exchanging data between the CU and the mobile device,wherein adapting the link layer context includes associating anuppermost protocol layer of the first DU with a lowest protocol layer ofthe CU.

The method also includes storing, by the first DU, third user data forthe CU received from the second DU, and sending, by the first DU, thethird user data to the CU. The first user data is sent after sending thesecond user data. The first DU is a target DU and the second DU is asource DU. Participating in the link layer context transfer includesreceiving the link layer context from the second DU. The link layercontext is for the mobile device.

In accordance with an example embodiment, a method for operating asecond DU is provided. The method includes participating, by the secondDU, in a link layer context transfer with a first DU, releasing, by thesecond DU, a user data path for a mobile device with a CU including aradio control protocol entity communicating with the mobile device,storing, by the second DU, first user data for the mobile devicereceived from the CU, receiving, by the second DU, a first indicationindicating that a connection with the mobile device is established, andsending, by the second DU, the first user data to the mobile device.

The method also includes storing, by the second DU, second user data forthe CU received from the mobile device, and sending, by the second DU,the second user data to the CU after receiving the first indication. Themethod also includes sending, by the second DU, a second indicationprompting the mobile device to transfer a link layer associated with themobile device to the first DU.

In accordance with an example embodiment, a method for operating a CU isprovided. The method includes participating, by the CU, in a release ofa first path between the CU and a first DU, storing, by the CU, firstuser data for a mobile device, participating, by the CU, in anestablishment of a second path between the CU and a second DU, andsending, by the CU, the first user data to the second DU.

The method also includes sending, by the CU, second user data for themobile device to the first DU prior to releasing the first path. Themethod also includes receiving, by the CU, third user data from thesecond DU.

In accordance with an example embodiment, a method for operating amobile device is provided. The method includes receiving, by the mobiledevice, an indication prompting the mobile device to transfer a linklayer associated with the mobile device to a first DU, participating, bythe mobile device, in a handover with the first DU and a second DU, andreceiving, by the mobile device, first user data from the second DU.

The method also includes sending, by the mobile device, second user datato the first DU prior to participating in the handover. The method alsoincludes receiving, by the mobile device, Layer 2 configurationinformation. The method also includes sending, by the mobile device,third user data to the second DU after participating in the handover.The Layer 2 configuration information includes information related to aconnection between the mobile device and the second DU.

In accordance with an example embodiment, a first DU is provided. Thefirst DU includes one or more processors, and a computer readablestorage medium storing programming for execution by the one or moreprocessors. The programming including instructions to configure thefirst DU to participate in a link layer context transfer with a secondDU, establish a user data path for a mobile device, the user data pathconnecting the mobile device with a CU including a radio controlprotocol entity communicating with the mobile device, store first userdata for the mobile device received from the CU, store second user datafor the mobile device received from the second DU, establish aconnection with the mobile device, send the first user data and thesecond user data to the mobile device, and adapt the link layer contextto operate in the first DU for exchanging data between the CU and themobile device, wherein adapting the link layer context includesassociating an uppermost protocol layer of the first DU with a lowestprotocol layer of the CU.

The programming includes instructions to configure the first DU to storethird user data for the CU received from the second DU, and send thethird user data to the CU. The programming includes instructions toconfigure the first DU to receiving the link layer context from thesecond DU.

Practice of the foregoing embodiments enables Layer 2 mobility of a userequipment between distributed units. Therefore, burdensome Layer 3signaling between the user equipment and the centralized unit isavoided.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 is an example communications system according to exampleembodiments described herein;

FIG. 2 is a high level view of an example communications systemaccording to example embodiments described herein;

FIG. 3 is a diagram of an example break before make Layer 2 switchingprocedure according to example embodiments described herein;

FIG. 4 illustrates a diagram of an example Layer 2 switching procedurehighlighting downlink data transmission according to example embodimentsdescribed herein;

FIG. 5 illustrates a diagram of an example Layer 2 switching procedurehighlighting the ordering of downlink SDUs according to exampleembodiments described herein;

FIG. 6 illustrates a diagram of an example Layer 2 switching procedurehighlighting uplink data transmission according to example embodimentsdescribed herein;

FIG. 7A illustrates a flow diagram of example operations occurring in asource DU participating in a Layer 2 switch highlighting downlinkcommunications according to example embodiments described herein;

FIG. 7B illustrates a flow diagram of example operations occurring in asource DU participating in a Layer 2 switch highlighting uplinkcommunications according to example embodiments described herein;

FIG. 8A illustrates a flow diagram of example operations occurring in atarget DU participating in a Layer 2 switch highlighting downlinkcommunications according to example embodiments described herein;

FIG. 8B illustrates a flow diagram of example operations occurring in atarget DU participating in a Layer 2 switch highlighting uplinkcommunications according to example embodiments described herein;

FIG. 9 illustrates a flow diagram of example operations 900 occurring ina CU participating in a Layer 2 switch according to example embodimentsdescribed herein;

FIG. 10 illustrates a flow diagram of example operations 1000 occurringin a UE participating in a Layer 2 switch according to exampleembodiments described herein;

FIG. 11 illustrates a block diagram of an embodiment processing systemfor performing methods described herein; and

FIG. 12 illustrates a block diagram of a transceiver adapted to transmitand receive signaling over a telecommunications network according toexample embodiments described herein.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The operating of the current example embodiments and the structurethereof are discussed in detail below. It should be appreciated,however, that the present disclosure provides many applicable inventiveconcepts that can be embodied in a wide variety of specific contexts.The specific embodiments discussed are merely illustrative of specificstructures of the embodiments and ways to operate the embodimentsdisclosed herein, and do not limit the scope of the disclosure.

FIG. 1 shows an example communications system 100. Communications system100 is a 5G cellular system. Communications system 100 includes aplurality of user equipments (UEs), such as UE 105, UE 107, and UE 109.Communications system 100 also includes distributed units (DUs), such asDU 110 and DU 112. Communications system 100 also includes an enhancedLTE (eLTE) evolved NodeB (eNB) 115 that serves UE log, for example,without utilizing the NR design. Communications system 100 also includesa remote centralized unit (CU) 120, which includes a user plane (UP)protocol stack 122 and a control plane (CP) protocol stack 124. CU 120is connected to the DUs, as well as eLTE eNB 115 and core network (CN)125.

While it is understood that communications systems may employ multiplenetwork entities capable of communicating with a number of UEs, only twoDUs and one eLTE eNB, and three UEs are illustrated for simplicity. Inaddition, it should be understood that the network topology in FIG. 1 isexemplary, and particular networks may embody different topologies. Forinstance, the DUs of a communications system might not connect directlyto the eNBs of an eLTE system (interface R3 in the figure), and some 5Gcellular systems could operate in a so-called “standalone” mode withoutinterworking with any neighbouring eLTE system. Therefore, thecommunications system shown in FIG. 1 should not be construed as beinglimiting to either the scope or spirit of the example embodiments.

As discussed previously, DUs implement a portion of the protocol stack.As shown in FIG. 1, the DUs include PHY sublayer, MAC sublayer, andradio link control (RLC) sublayer entities. As an illustrative example,DU 110 includes PHY sublayer entity 130, MAC sublayer entity 132, andRLC sublayer entity 134. The CUs and UPs also implement a portion of theprotocol stack. As shown in FIG. 1, the CUs and the UPs implement IP andPDCP layer/sublayer entities in the user plane, and RRC and PDCPlayer/sublayer entities in the control plane. As an illustrativeexample, the control plane protocol stack 124 of CU 120 includesRRC/PDCP sublayer entity 135. Although the RRC and PDCP entities areshown as a combined entity 135, they may be implemented as a singleentity or as separate entities, depending on the implementation of theCU.

Communications system 100, as shown in FIG. 1, illustrates ahierarchical model of the NR design of a 5G cellular system, with one CUmanaging many DUs (e.g., CU 120 managing DUs 110 and 112). Thehierarchical model shown in FIG. 1 illustrates a centralized CU anddistributed DUs. Although the NR design may be extended to situationswith more than one CU, FIG. 1 focuses on the portion of an NR radioaccess network under the management of one CU. In general, a UE isserved by a DU, and as a UE moves around, the link between UE and DU isrelocated or switched to different DUs. It is noted that Layer 3 controlis located at the CU. As a result, the relocation of the UE's link to adifferent DU under the control of the same CU does not require acorresponding relocation of the layer 3 anchor point for the UE.

However, the locating of the Layer 3 in the CU may lead to long latencyfor air interface signaling when layer 3 procedures are used. Thesignaling exchange between the CU and the DU is not amenable insituations that involve delay sensitive signaling or services. Themagnitude of delay involved depends on various factors such as thetransport used for the CU-DU interface (R1-C/R1-U interface in FIG. 1),the amount of network load managed by the CU, etc.

Layer 2 protocol sublayers are split between the CU and the DU so that afirst sublayer of packet processing (the PDCP sublayer) as well assecurity is located in the CU and the MAC and PHY sublayers are in theDU. The reliability sublayer (the RLC sublayer) may be located in eitherthe CU or the DU (although shown being located in the DU in FIG. 1). Insome situations, the locating of the RLC sublayer in the CU, or evendividing RLC functionality between the CU and DU, may be effective.Other layers or sublayers may be present, but are not shown.

The discussion presented herein utilizes the following terms:

-   -   Transmission-reception point (TRP): a device that is capable of        transmitting and receiving, such as a DU, an eNB. A TRP may also        be referred to as a remote radio unit (RRU).    -   CU: a central entity for control, for both control plane (CP)        and UP. The protocol layers located in the CU comprise the        controlling protocol layer for CP functions, e.g. an RRC        protocol sublayer, as well as the layer/sublayer that controls        transport functions for the user plane, e.g. a PDCP sublayer.        Logically, a single CU can manage one or multiple cell anchor        points, such as DUs, eNBs, gNBs (the accepted acronym for the NR        equivalent of eNBs), and so on. RRC/PDCP sublayers are located        in the CU.    -   DU: a distributed entity for radio deployment. One DU may be        connected to one or multiple RRUs or TRPs. MAC/PHY sublayers are        located in the DU, as well as possibly the RLC layer.    -   Physical cell: a sector of a TRP/RRU or a cluster of one or more        TRP/RRUs coordinated to appear as a single object in radio        layers. As defined for a traditional cell, the identity of a        physical cell, e.g., a physical cell identity (PCI), is unique        in a limited coverage area.    -   Cell anchor point: a Layer 3 concept of cell at the RRC layer        and NG-C interface. The cell anchor point is also referred to as        a Layer 3 anchor. One cell anchor point can be mapped to        multiple physical cells (as defined in Layer 2). A global cell        identifier (e.g., a cell global identity (CGI) in 3GPP LTE) may        be defined at the Layer 3 anchor.

In current generation cellular systems, a cell is shared across Layers1, 2, and 3. At Layers 1 and 2, the cells are distinguished by theirPCI, while the CGI is used to identify the cells in Layer 3. However,the PCI and the CGI define the same scope in 3GPP LTE, that is, aparticular LTE cell has exactly one PCI and exactly one CGI. Theidentifiers are different only to allow for a longer globally uniqueidentifier in Layer 3.

According to an example embodiment, the cell structure is decoupledbetween Layers 2 and 3. It is assumed that the Layer 1 cell or coveragearea is transparent to Layers 2 and 3. The cell concept at Layer 2 isreferred to as a physical cell. Because the physical cell in 3GPP LTE isdistinguished by the PCI, the Layer 2 is the lowest level where a cellis visible in higher layer protocols.

According to an example embodiment, a hierarchical model is presentedwith a Layer 3 cell (e.g., a CU) managing multiple Layer 2 cells (e.g.,DUs). Therefore, the cell structure is decoupled between Layers 2 and 3.Although the term cell is used, other terms, such as anchor point, DUarea, and so on, may be used. FIG. 2 illustrates a high level view of anexample communications system 200. Communications system 200 includes aLayer 3 cell 205. Layer 3 cell 205 comprises a CU 210 managing aplurality of Layer 2 cells. Each Layer 2 cell is implemented as a DU. Asan example, Layer 2 cell 215 is implemented as DU 220. In somedeployments, a CU comprises multiple Layer 3 cells, each with its owncomplement of Layer 2 cells.

According to an example embodiment, at least some of the functionalitythat in current cellular systems would be embodied in RRC controlsignaling is relocated to DUs (Layer 2). The control signaling at theDUs may be performed through signalling at any of various Layer 2sublayers, e.g., the RLC or MAC sublayers, to speed up the procedure andto avoid unnecessary signaling overhead. Furthermore, no Layer 3 RRChandling at the CU is involved as long as the DUs can handle thecoordination between the DUs and UEs at Layer 2 to support the concernedfunctionality. Additionally, consideration of user plane aspects isprovided. As an example, handovers between physical cells (Layer 2mobility) while cell anchor point remains the same (avoiding Layer 3mobility) are considered. Table 1 illustrates 3GPP LTE RRC functionswith equivalent 5G functions, showing a division into 5G RRC (Layer 3)procedures and 5G Layer 2 procedures. For example, as shown in Table 1,the LTE handover procedure may be realised separately in Layer 2 andLayer 3 in a 5G system: A handover (physical cell change) may beimplemented via a Layer 2 cell change (L2CC) when the mobility affectsthe Layer 2 anchor point, e.g., DU, and additionally may be implementedvia a Layer 3 RRC connection reconfiguration when the Layer 3 anchorpoint, e.g., CU, is changed.

TABLE 1 3GPP LTE RRC Functions with Equivalent 5G Functions. LTE RRCfunctions LTE RRC ->5G RRC LTE RRC ->5G L2 System information X Possible(e.g. specific SIB for L2 configuration) Radio resource configuration XPossible (e.g. Neighbour Cell List, MAC configuration) Paging X RRCconnection establishment X X RRC connection re-establishment (L3connection control) (L2 connection control) RRC connection release RRCconnection reconfiguration X X with mobility control (Handover) (L3 CellChange) (Layer 2 Cell Change) Radio link failure related actions X(Detection of RLF) Measurements X X (L3 Measurements) (L2 Measurements)Security X Possible (L2 security procedures) Inter-RAT mobility X RRCstate manangement X (including Idle) MBMS X other L3 procedure X (e.g.Mobility history info)

According to an example embodiment, the Layer 2 relocation decision ismade in the network (e.g., DUs, CU, CN, and so on) based on Layer 2and/or Layer 3 measurements. The measurements may be made by the networkbased on uplink signals, made by the UE based on downlink signals andreported to the network, or a combination of the two. The Layer 2relocation may involve one or more DUs, the CU, and so on, but thedetails are transparent to the UE As an example, a source DU (the DUthat the UE is currently attached to) makes a decision on inter-DUswitching. The decision power of a DU may need to be authorized a prioriby the CU. In a situation when a decision regarding DU switching ismade, the DU reports the decision to the CU to prepare the CU for datadelivery path switching associated with Layer 2 switching. Other nodessuch as potential target DUs, one or more CUs, etc. may also contributeto the decision on switching.

An example Layer 2 switching procedure includes:

-   -   Lower layer context is passed from the source DU to the target        DU (the DU that the UE is to become attached to after completion        of the Layer 2 relocation). The source DU transfers the RLC        context for the radio link to the target DU, for example.    -   The data delivery path switches from source DU<->CU to target        DU<->CU.    -   Layer 2 switching is ordered to the UE from the source DU using        Layer 2 control signalling, e.g., a MAC control element (MAC        CE). The UE is triggered to begin communications with the target        DU, using the same Layer 2 configuration as used with the source        DU modified according to any changes in the Layer 2        configuration indicated in the Layer 2 switch order, for        example.    -   Airlink data communications begin when the UE is detected by the        target DU, such as during an access procedure.

Variations of the Layer 2 switching procedure include a break beforemake procedure where the connection with the source DU is broken priorto the establishment of the connection with the target DU, as well as amake before break procedure, where the connection with the target DU ismade before the connection with the source DU is broken. For the Layer 2switching procedures discussed herein, it is assumed that the RLC islocated in the DU; therefore, the data packets exchanged between the CUand the DUs are RLC service data units (SDUs), which are equivalent toPDCP protocol data units (PDUs). Alternatively, if the RLC is in the CU,the packets would be MAC SDUs, which are equivalent to RLC PDUs. Ingeneral, the data consists of SDUs of the top layer or sublayer in theDU, which are PDUs of the bottom layer or sublayer in the CU.

FIG. 3 illustrates a diagram 300 of an example break before make Layer 2switching procedure. Diagram 300 illustrates messages exchanged andprocessing performed by a UE 305, a target DU 307, a source DU 309, anda CU 311 participating in a break before make Layer 2 switchingprocedure. Diagram 300 also illustrates protocol stacks at the variousdevices and relationships between the protocol stacks.

A relocation decision is made based on link monitoring and measurementsthereof (block 315). The relocation decision may be made by source DU309, target DU 307, and/or CU 311. As an illustrative example, networkmonitoring of uplink signals transmitted by UE 305 is used as acriterion for the relocation decision. Source DU 309 and target DU 307participate in a context transfer (event 317). Source DU 309 and CU 311participate in a Layer 2 path release (event 319). After the contexttransfer completes, source DU 309 stops delivering data from UE 305.Instead, source DU 309 buffers uplink data for later forwarding (block321). At substantially the same time, CU 311 stops delivering data to UE305. Instead, CU 311 buffers downlink data for later forwarding (block323). In between the start of event 317 and the end of event 319,downlink data for UE 305 may arrive at source DU 309. Source DU 309cannot deliver such data because it has already begun to transfer theLayer 2 context to target DU 307; instead, source DU 309 buffers thedownlink data and forwards the downlink data to target DU 307 at a latertime, after completion of the handover, for example. Diagram 300 doesnot illustrate the arrival of the downlink data, the buffering of thedownlink data, or the forwarding of the downlink data. The amount ofdownlink data to be buffered and forwarded in this way would be expectedto be small, since the procedures of steps 317 and 319 should preferablybe as nearly simultaneous as possible. Similarly, because the connectionbetween UE 305 and target DU 307 has not been established at this point,CU 311 buffers downlink data intended for UE 305 (block 323) and laterforwards it to target DU 307 as described below (event 327).

CU 311 and target DU 307 participate in establishing a Layer 2 path(event 325). With the Layer 2 path established between CU 311 and targetDU 307, CU 311 forwards downlink data buffered by CU 311 to target DU307 (event 327). Source DU 309 sends an airlink triggering event, suchas a Layer 2 switch instruction (e.g., a MAC CE) to UE 305 (event 329).The airlink triggering event may be optional because the Layer 2 switchmay occur without the airlink triggering event, e.g., based on anautonomous decision by the UE, on a UE behaviour controlled bypreviously configured parameters determined by the network, etc.Activity between UE 305 and target DU 307 (event 331) occurs, such as arandom access procedure, an uplink transmission by the UE, etc., andtarget DU 307 sends a handover complete indication to source DU 309(event 333). Target DU 307 sends downlink data to UE 305 (event 335).The handover complete indication at event 333 may be an indication tosource DU 309 that UE 305 has been detected by, or become connected to,target DU 307. The handover complete indication is not assumed to be aforwarded message from UE 305. In this manner, the example embodimentdiffers from a 3GPP LTE handover.

With the handover complete, source DU 309 stops buffering uplink data(block 337) and sends buffered uplink data to target DU 307 (event 339)for subsequent forwarding to CU 311 (event 341). Alternatively, sourceDU 309 may forward the buffered uplink data directly to CU 311, but thisapproach has a cost in potential extra data transmission resulting froma Layer 2 context misalignment. It is noted that between events 317 and339, the Layer 2 and Layer 1 contexts are out of sync and that theforwarding of the uplink data to target DU 307 realigns the contexts,e.g., by allowing the reliability layer (for instance, an RLC sublayer)to process requests, acknowledgements, any other control informationthat may be present in the uplink data stored by source DU 309. Inaddition, forwarding the stored uplink data to target DU 307 allows thelatter to know when all stored uplink data have been delivered and newuplink data from the UE can be sent to CU 311, without potentiallycausing out of order arrivals. The flow of uplink data from UE 305 to CU311 is interrupted from event 317 to event 341 in the figure.

FIG. 4 illustrates a diagram 400 of an example Layer 2 switchingprocedure highlighting downlink data transmission. Diagram 400illustrates messages exchanged and processing performed by a UE 405, atarget DU 407, a source DU 409, and a CU 411 participating in a Layer 2switch procedure. Block 415 illustrates the Layer 2 data path before theLayer 2 switch takes place.

A relocation decision is made based on link monitoring and measurementsthereof (block 417). The relocation decision may be made by source DU409, target DU 407, and/or CU 411. Source DU 409 and target DU 407participate in a context transfer (event 419). After completion of thecontext transfer, downlink data are sent from CU 411 to source DU 423(event 421) in the form of RLC SDUs (block 423). Source DU 409 sends thedownlink data to target DU 407 (event 425) where it is buffered in theform of RLC SDUs (block 427). CU 411 and source DU 409 participate in aLayer 2 path release procedure (event 429). After completion of theLayer 2 path release procedure and before a new Layer 2 path isestablished between CU 411 and target DU 407, CU 411 buffers anydownlink data intended for UE 405 in the form of PDCP SDUs or PDCP PDUs(block 431). It is noted that CU 411 receives PDCP SDUs and can eitherbuffer the PDCP SDUs and then convert them into PDCP PDUs prior tosending the PDCP PDUs to target DU 407 or CU 411 can convert thereceived PDCP SDUs into PDCP PDUs and then buffer the PDCP PDUs prior tosending the PDCP PDUs to target DU 407. In some embodiments, the Layer 2path release procedure (event 429) and the Layer 2 path establishment(event 433) may be considered as a single “path switch” procedure. Aftercompletion of the Layer 2 path release procedure of event 429, source DU409 should no longer receive downlink data from CU 411. It may bepossible for source DU 409 to complete downlink data forwarding prior toevent 441 instead of waiting until event 445. However, there is noimpact upon handover performance in either situation.

CU 411 and target DU 407 participate in the establishing of a Layer 2path (event 433). With the Layer 2 path established, CU 411 sends thebuffered PDCP PDUs (block 435) to target DU 407 (event 437) where theyare buffered (block 439). It is noted that it may be necessary toreorder the buffered RLC SDUs, which may have been received from sourceDU 409 and CU 411. The last SDU received from source DU 409 should bedelivered prior to the delivery of the first SDU from CU 411. CU 411 cancontinue to deliver SDUs to source DU 409 (prior to event 429) andtarget DU 407 (after event 433).

Source DU 409 optionally sends an airlink triggering event, such as aLayer 2 switch instruction (e.g., a MAC CE) to UE 405 (event 441).Activity between UE 405 and target DU 407 (event 443) occurs and targetDU 407 sends a handover complete indication to source DU 409 (event445). Source DU 409 sends any remaining RLC SDUs (block 447) to targetDU 407 (event 449), which sends the downlink data to UE 405 (event 451).Block 453 illustrates the Layer 2 data path after the Layer 2 switchtakes place.

Events 443 and 445, as shown in FIG. 4, are based on 3GPP LTE.Alternative ways for completing the handover procedure are possible. Aslong as target DU 407 knows when UE 405 arrives and source DU 409 knowswhen UE 405 is acquired by target DU 407, a variety of differentapproaches are possible. Therefore, the 3GPP LTE based technique shouldnot be construed as being limiting to either the scope or spirit of theexample embodiments.

FIG. 5 illustrates a diagram 500 of an example Layer 2 switchingprocedure highlighting the ordering of downlink SDUs. Diagram 500illustrates messages exchanged and processing performed by a UE 505, atarget DU 507, a source DU 509, and a CU 511 participating in a Layer 2switching procedure.

Because target DU 507 can receive downlink data, e.g., RLC SDUs, fromsource DU 509 or CU 511 depending on when the downlink data is receivedrelative to the Layer 2 path switch, it may be necessary for target DU507 to take actions to deliver the downlink data in order. According toan example embodiment, downlink data from source DU 509 is alwaysordered for delivery to the UE before downlink data from CU 511,irrespective of the order in which the data arrive at target DU 507.Therefore, target DU 507 buffers the downlink data from both sourcesseparately until source DU 509 indicates that it has forwarded the lastSDU. As an example, source DU 509 can indicate that the last downlinkdata have been forwarded at event 515, which occurs before thecompletion of the handover (event 519) or event 517, which occurs afterthe completion of the handover (event 519). After target DU 507 receivesthe indication from source DU 509, then target DU 507 sends buffereddownlink data from source DU 509 (event 521) followed by buffereddownlink data from CU 511 (event 523).

FIG. 6 illustrates a diagram 600 of an example Layer 2 switchingprocedure highlighting uplink data transmission. Diagram 600 illustratesmessages exchanged and processing performed by a UE 605, a target DU607, a source DU 609, and a CU 611 participating in a Layer 2 switchprocedure. Block 615 illustrates the Layer 2 data path before the Layer2 switch takes place.

A relocation decision is made based on link monitoring and measurementsthereof (block 617). The relocation decision may be made by source DU609, target DU 607, and/or CU 611. Source DU 609 and target DU 607participate in a context transfer (event 619). Source DU 509 and CU 611participate in a Layer 2 path release (event 621). After completion ofthe Layer 2 path release, UE 605 may continue to send uplink data tosource DU 609 (event 623). However, since the Layer 2 path has beenreleased, no more uplink data can be sent to CU 611 and source DU 6009buffers the data from UE 605 in the form of RLC PDUs (block 625). SourceDU 609 buffers the RLC PDUs from UE 605 until a Layer 2 switch withtarget DU 607 is completed.

Target DU 607 and CU 611 participate in a Layer 2 path establishment(event 627). Source DU 609 optionally sends an airlink triggering event,such as a Layer 2 switch (e.g., a MAC CE) to UE 6005 (event 629).Activity between UE 605 and target DU 607 (event 631) occurs and targetDU 607 sends a handover complete indication to source DU 609 (event633). Source DU 609 sends buffered uplink data to target DU 607 (event635). The buffered uplink data is sent to target DU 607 because targetDU 607 has a Layer 2 path with CU 611. Furthermore, because target DU607 has all of the uplink data from source DU 609, target DU 607 knowswhen all of the uplink data from source DU 609 has been sent to CU 611before target DU 607 sends uplink data from UE 605. The control bytarget DU 607 prevents CU 611 from having to reorder the uplink data.Finally, the forwarding of RLC PDUs to target DU 607 allows the PDUs tobe processed by an RLC protocol entity in target DU 607, which can carryout procedures requiring an airlink connection to the UE, e.g.,delivering to the UE acknowledgements for PDUs that were transmittedusing an RLC acknowledged mode (RLC AM).

Target DU 607 sends the uplink data to CU 611 (event 637). Target DU 607sends Layer 2 configuration information to UE 605 (event 639). The Layer2 configuration information provides UE 605 with information to enableUE 605 to send uplink information to target DU 6005. UE 605 sends uplinkdata to target DU 607 (event 641). Target DU 607 buffers uplink datafrom UE 6005 until all uplink data from source DU 609 has been sent toCU 611 (block 643), at which time, target DU 607 sends the uplink datafrom UE 605 to CU 611 (event 645). Block 645 illustrates the Layer 2data path after the Layer 2 switch takes place.

It is noted that delivering the uplink data directly to CU 611 ispossible. However, CU 611 would have to ensure the correct ordering ofthe RLC SDUs, which implies reordering in the PDCP sublayer or an upperRLC sublayer located in CU 611.

FIG. 7A illustrates a flow diagram of example operations 700 occurringin a source DU participating in a Layer 2 switch highlighting downlinkcommunications. Operations 700 may be indicative of operations occurringin a source DU as the source DU participates in a Layer 2 switch.

Operations 700 begin with the source DU participating in a relocationdecision (block 705). The relocation decision may be made withparticipation of the source DU, a target DU, and a CU. The relocationdecision may be made in accordance with measurements of links. Thesource DU participates in a context transfer with the target DU (block707). The source DU buffers downlink data from the CU (block 709) andsends the buffered downlink data to the target DU (block 711). Thesource DU and the CU exchange messaging to release the Layer 2 pathbetween the source DU and the CU (block 713). The source DU mayoptionally send an airlink triggering event. The source DU completes thehandover (block 715). Completing the handover may involve the source DUdetecting activity between a UE and the target DU and the source DUreceiving a handover complete indication from the target DU. The sourceDU forwards buffered downlink data to the target DU (block 717).

FIG. 7B illustrates a flow diagram of example operations 750 occurringin a source DU participating in a Layer 2 switch highlighting uplinkcommunications. Operations 750 may be indicative of operations occurringin a source DU as the source DU participates in a Layer 2 switch.

Operations 750 begin with the source DU participating in a relocationdecision (block 755). The relocation decision may be made withparticipation of the source DU, a target DU, and a CU. The relocationdecision may be made in accordance with measurements of links. Thesource DU participates in a context transfer with the target DU (block757). The source DU and the CU exchange messaging to release the Layer 2path between the source DU and the CU (block 759). The source DU buffersuplink data from the UE (block 761). The source DU may optionally sendan airlink triggering event. The source DU completes the handover (block763). Completing the handover may involve the source DU detectingactivity between a UE and the target DU and the source DU receiving ahandover complete indication from the target DU. The source DU sendsbuffered uplink data to the target DU (block 765).

In a first aspect, the present application provides a method foroperating a first DU. The method includes participating, by the firstDU, in a link layer context transfer with a second DU, establishing, bythe first DU, a user data path for a mobile device, the user data pathconnecting the mobile device with a CU including a radio controlprotocol entity communicating with the mobile device, storing, by thefirst DU, first user data for the mobile device received from the CU,storing, by the first DU, second user data for the mobile devicereceived from the second DU, establishing, by the first DU, a connectionwith the mobile device, sending, by the first DU, the first user dataand the second user data to the mobile device, and adapting, by thefirst DU, the link layer context to operate in the first DU forexchanging data between the CU and the mobile device, wherein adaptingthe link layer context includes associating an uppermost protocol layerof the first DU with a lowest protocol layer of the CU.

According to a first embodiment of the method according to the firstaspect, the method includes storing, by the first DU, third user datafor the CU received from the second DU, and sending, by the first DU,the third user data to the CU. According to a second embodiment of themethod according to any preceding embodiment of the first aspect or thefirst aspect as such, the first user data is sent after sending thesecond user data. According to a third embodiment of the methodaccording to any preceding embodiment of the first aspect or the firstaspect as such, the first DU is a target DU and the second DU is asource DU. According to a fourth embodiment of the method according toany preceding embodiment of the first aspect or the first aspect assuch, participating in the link layer context transfer includesreceiving the link layer context from the second DU. According to afifth embodiment of the method according to any preceding embodiment ofthe first aspect or the first aspect as such, the link layer context isfor the mobile device.

FIG. 8A illustrates a flow diagram of example operations 800 occurringin a target DU participating in a Layer 2 switch highlighting downlinkcommunications. Operations 800 may be indicative of operations occurringin a target DU as the target DU participates in a Layer 2 switch.

Operations 800 begin with the target DU participating in a relocationdecision (block 805). The relocation decision may be made withparticipation of a source DU, the target DU, and a CU. The relocationdecision may be made in accordance with measurements of links. Thetarget DU participates in a context transfer with the source DU (block807). The target DU buffers downlink data (block 809). The target DU andthe CU exchange messaging to establish a Layer 2 path between the targetDU and the CU (block 811). The target DU buffers downlink data from theCU (block 813). The target DU completes the handover (block 815).Completing the handover may include the target DU participating incommunications with the UE and sending a handover complete indication tothe source DU. The target DU sends buffered downlink data to the UE(block 809).

FIG. 8B illustrates a flow diagram of example operations 850 occurringin a target DU participating in a Layer 2 switch highlighting uplinkcommunications. Operations 850 may be indicative of operations occurringin a target DU as the target DU participates in a Layer 2 switch.

Operations 850 begin with the target DU participating in a relocationdecision (block 855). The relocation decision may be made withparticipation of a source DU, the target DU, and a CU. The relocationdecision may be made in accordance with measurements of links. Thetarget DU participates in a context transfer with the source DU (block857). The target DU and the CU exchange messaging to establish a Layer 2path between the target DU and the CU (block 869). The target DUcompletes the handover (block 871). Completing the handover may includethe target DU participating in communications with the UE and sending ahandover complete indication to the source DU. The target DU buffersuplink data from the source DU as well as from the UE (block 863). Thetarget DU sends the buffered uplink data to the CU (block 865).

In a second aspect, the present application provides a method foroperating a second DU. The method includes participating, by the secondDU, in a link layer context transfer with a first DU, releasing, by thesecond DU, a user data path for a mobile device with a CU including aradio control protocol entity communicating with the mobile device,storing, by the second DU, first user data for the mobile devicereceived from the CU, receiving, by the second DU, a first indicationindicating that a connection with the mobile device is established, andsending, by the second DU, the first user data to the mobile device.

According to a first embodiment of the method according to the secondaspect, the method includes storing, by the second DU, second user datafor the CU received from the mobile device, and sending, by the secondDU, the second user data to the CU after receiving the first indication.According to a second embodiment of the method according to anypreceding embodiment of the second aspect or the second aspect as such,the method includes sending, by the second DU, a second indicationprompting the mobile device to transfer a link layer associated with themobile device to the first DU.

FIG. 9 illustrates a flow diagram of example operations 900 occurring ina CU participating in a Layer 2 switch. Operations 900 may be indicativeof operations occurring in a CU as the CU participates in a Layer 2switch.

Operations 900 begin with the CU sending downlink data to a source DUfor delivery to a UE (block 905). The CU participates in a release of apath between the CU and the source DU (block 910). Because the path hasbeen released, the CU buffers any additional downlink data intended forthe UE (block 915). The CU participates in establishing a path betweenthe CU and a target DU (block 920). The CU sends buffered downlink datato the target DU for delivery to the UE (block 925). The CU receivesuplink data (block 930).

In a third aspect, the present application provides a method foroperating a CU. The method includes participating, by the CU, in arelease of a first path between the CU and a first DU, storing, by theCU, first user data for a mobile device, participating, by the CU, in anestablishment of a second path between the CU and a second DU, andsending, by the CU, the first user data to the second DU.

According to a first embodiment of the method according to the thirdaspect, the method includes sending, by the CU, second user data for themobile device to the first DU prior to releasing the first path.According to a second embodiment of the method according to anypreceding embodiment of the third aspect or the third aspect as such,the method includes receiving, by the CU, third user data from thesecond DU.

Figure to illustrates a flow diagram of example operations 1000occurring in a UE participating in a Layer 2 switch. Operations 1000 maybe indicative of operations occurring in a UE as the UE participates ina Layer 2 switch.

Operations 1000 begin with the UE sending uplink data to a source DU,the uplink data intended for a CU (block 1005). The UE receives anairlink triggering event, such as a Layer 2 switch instruction (e.g., aMAC CE) (block 1010). The UE participates in a handover between thesource DU and the target DU (block 1015). The UE receives Layer 2configuration information for a connection between the target DU and theUE (block 1020). The UE receives downlink data from the target DU (block1025). The UE sends uplink data to the target DU, the uplink dataintended for the CU (block 1030).

In a fourth aspect, the present application provides a method foroperating a mobile device. The method includes receiving, by the mobiledevice, an indication prompting the mobile device to transfer a linklayer associated with the mobile device to a first DU, participating, bythe mobile device, in a handover with the first DU and a second DU, andreceiving, by the mobile device, first user data from the second DU.

According to a first embodiment of the method according to the fourthaspect, the method includes sending, by the mobile device, second userdata to the first DU prior to participating in the handover. Accordingto a second embodiment of the method according to any precedingembodiment of the fourth aspect or the fourth aspect as such, the methodincludes receiving, by the mobile device, Layer 2 configurationinformation. According to a third embodiment of the method according toany preceding embodiment of the fourth aspect or the fourth aspect assuch, the method includes sending, by the mobile device, third user datato the second DU after participating in the handover. According to afourth embodiment of the method according to any preceding embodiment ofthe fourth aspect or the fourth aspect as such, the Layer 2configuration information comprises information related to a connectionbetween the mobile device and the second DU.

In a fifth aspect, the present application provides a first DU. Thefirst DU includes one or more processors, and a computer readablestorage medium storing programming for execution by the one or moreprocessors. The programming including instructions to configure thefirst DU to participate in a link layer context transfer with a secondDU, establish a user data path for a mobile device, the user data pathconnecting the mobile device with a CU including a radio controlprotocol entity communicating with the mobile device, store first userdata for the mobile device received from the CU, store second user datafor the mobile device received from the second DU, establish aconnection with the mobile device, send the first user data and thesecond user data to the mobile device, and adapt the link layer contextto operate in the first DU for exchanging data between the CU and themobile device, wherein adapting the link layer context includesassociating an uppermost protocol layer of the first DU with a lowestprotocol layer of the CU.

According to a first embodiment of the first DU according to the fifthaspect, the programming includes instructions to configure the first DUto store third user data for the CU received from the second DU, andsend the third user data to the CU. According to a second embodiment ofthe first DU according to any preceding embodiment of the fifth aspector the fifth aspect as such, the programming includes instructions toconfigure the first DU to receiving the link layer context from thesecond DU.

FIG. 11 illustrates a block diagram of an embodiment processing system1100 for performing methods described herein, which may be installed ina host device. As shown, the processing system 1100 includes a processor1104, a memory 1106, and interfaces 1110-1114, which may (or may not) bearranged as shown in FIG. 11. The processor 1104 may be any component orcollection of components adapted to perform computations and/or otherprocessing related tasks, and the memory 1106 may be any component orcollection of components adapted to store programming and/orinstructions for execution by the processor 1104. In an embodiment, thememory 1106 includes a non-transitory computer readable medium. Theinterfaces 1110, 1112, 1114 may be any component or collection ofcomponents that allow the processing system 1100 to communicate withother devices/components and/or a user. For example, one or more of theinterfaces 1110, 1112, 1114 may be adapted to communicate data, control,or management messages from the processor 1104 to applications installedon the host device and/or a remote device. As another example, one ormore of the interfaces 1110, 1112, 1114 may be adapted to allow a useror user device (e.g., personal computer (PC), etc.) tointeract/communicate with the processing system 1100. The processingsystem 1100 may include additional components not depicted in FIG. 11,such as long term storage (e.g., non-volatile memory, etc.).

In some embodiments, the processing system goo is included in a networkdevice that is accessing, or part otherwise of, a telecommunicationsnetwork. In one example, the processing system 1100 is in a network-sidedevice in a wireless or wireline telecommunications network, such as abase station, a relay station, a scheduler, a controller, a gateway, arouter, an applications server, or any other device in thetelecommunications network. In other embodiments, the processing system1100 is in a user-side device accessing a wireless or wirelinetelecommunications network, such as a mobile station, a user equipment(UE), a personal computer (PC), a tablet, a wearable communicationsdevice (e.g., a smartwatch, etc.), or any other device adapted to accessa telecommunications network.

In some embodiments, one or more of the interfaces 1110, 1112, 1114connects the processing system 1100 to a transceiver adapted to transmitand receive signaling over the telecommunications network. FIG. 12illustrates a block diagram of a transceiver 1200 adapted to transmitand receive signaling over a telecommunications network. The transceiver1200 may be installed in a host device. As shown, the transceiver 1200comprises a network-side interface 1202, a coupler 1204, a transmitter1206, a receiver 1208, a signal processor 1210, and a device-sideinterface 1212. The network-side interface 1202 may include anycomponent or collection of components adapted to transmit or receivesignaling over a wireless or wireline telecommunications network. Thecoupler 1204 may include any component or collection of componentsadapted to facilitate bi-directional communication over the network-sideinterface 1202. The transmitter 1206 may include any component orcollection of components (e.g., up-converter, power amplifier, etc.)adapted to convert a baseband signal into a modulated carrier signalsuitable for transmission over the network-side interface 1202. Thereceiver 1208 may include any component or collection of components(e.g., down-converter, low noise amplifier, etc.) adapted to convert acarrier signal received over the network-side interface 1202 into abaseband signal. The signal processor 1210 may include any component orcollection of components adapted to convert a baseband signal into adata signal suitable for communication over the device-side interface(s)1212, or vice-versa. The device-side interface(s) 1012 may include anycomponent or collection of components adapted to communicatedata-signals between the signal processor 1210 and components within thehost device (e.g., the processing system 1100, local area network (LAN)ports, etc.).

The transceiver 1200 may transmit and receive signaling over any type ofcommunications medium. In some embodiments, the transceiver 1200transmits and receives signaling over a wireless medium. For example,the transceiver 1200 may be a wireless transceiver adapted tocommunicate in accordance with a wireless telecommunications protocol,such as a cellular protocol (e.g., long-term evolution (LTE), etc.), awireless local area network (WLAN) protocol (e.g., Wi-Fi, etc.), or anyother type of wireless protocol (e.g., Bluetooth, near fieldcommunication (NFC), etc.). In such embodiments, the network-sideinterface 1202 comprises one or more antenna/radiating elements. Forexample, the network-side interface 1202 may include a single antenna,multiple separate antennas, or a multi-antenna array configured formulti-layer communication, e.g., single input multiple output (SIMO),multiple input single output (MISO), multiple input multiple output(MIMO), etc. In other embodiments, the transceiver 1200 transmits andreceives signaling over a wireline medium, e.g., twisted-pair cable,coaxial cable, optical fiber, etc. Specific processing systems and/ortransceivers may utilize all of the components shown, or only a subsetof the components, and levels of integration may vary from device todevice.

It should be appreciated that one or more steps of the embodimentmethods provided herein may be performed by corresponding units ormodules. For example, a signal may be transmitted by a transmitting unitor a transmitting module. A signal may be received by a receiving unitor a receiving module. A signal may be processed by a processing unit ora processing module. Other steps may be performed by a transferringunit/module, a storing unit/module, an establishing unit/module, and anadapting unit/module. The respective units/modules may be hardware,software, or a combination thereof. For instance, one or more of theunits/modules may be an integrated circuit, such as field programmablegate arrays (FPGAs) or application-specific integrated circuits (ASICs).

Although the present disclosure and its advantages have been describedin detail, it should be understood that various changes, substitutionsand alterations can be made herein without departing from the spirit andscope of the disclosure as defined by the appended claims.

What is claimed is:
 1. A method for operating a mobile device, themethod comprising: receiving, by the mobile device, an indicationprompting the mobile device to transfer a link layer associated with themobile device to a second distributed unit (DU); participating, by themobile device, in a handover of the link layer from a first DU to thesecond DU, while maintaining communication with a radio control protocolentity of a first centralized unit (CU) before and after the handover;and receiving, by the mobile device, first user data from the first DU.2. The method of claim 1, further comprising sending, by the mobiledevice, second user data to the second DU prior to participating in thehandover.
 3. The method of claim 1, further comprising receiving, by themobile device, Layer 2 configuration information.
 4. The method of claim3, wherein the Layer 2 configuration information comprises informationrelated to a connection between the mobile device and the first DU. 5.The method of claim 3, further comprising sending, by the mobile device,third user data to the first DU after participating in the handover. 6.A mobile device comprising: a non-transitory memory storage comprisinginstructions; and a processor in communication with the memory storage,wherein the processor executes the instructions to: receive anindication prompting the mobile device to transfer a link layerassociated with the mobile device to a second distributed unit (DU);participate in a handover of the link layer from a first DU to thesecond DU, while maintaining communication with a radio control protocolentity of a first centralized unit (CU) before and after the handover;and receive first user data from the first DU.
 7. The mobile device ofclaim 6, wherein the processor executes the instructions to send seconduser data to the second DU prior to participating in the handover. 8.The mobile device of claim 6, wherein the processor executes theinstructions to receive Layer 2 configuration information.
 9. The mobiledevice of claim 8, wherein the Layer 2 configuration informationcomprises information related to a connection between the mobile deviceand the first DU.
 10. The mobile device of claim 8, wherein theprocessor executes the instructions to send third user data to the firstDU after participating in the handover.
 11. A method comprising:communicating, by a mobile device, first user data with a radio controlprotocol entity of a centralized unit (CU) via a first link layer entityof a first distributed unit (DU), the first link layer entity associatedwith the radio control protocol entity; receiving, by the mobile device,an indication for the mobile device to transfer a link layer context toa second distributed unit (DU); participating, by the mobile device, ina handover from the first DU to the second DU; and communicating, by themobile device, second user data with the radio control protocol entityof the CU via a second link layer entity of the second DU, the secondlink layer entity associated with the radio control protocol entity. 12.The method of claim 11, the communicating the second user datacomprising receiving, by the mobile device, a portion of the second userdata from the CU via the first DU interposed between the CU and thesecond DU.
 13. The method of claim 11, further comprising receiving, bythe mobile device, Layer 2 configuration information related to aconnection between the mobile device and the first DU.
 14. The method ofclaim 11, the communicating the first user data comprising sending, bythe mobile device, a portion of the first user data to the CU via thefirst DU after participating in the handover.
 15. The method of claim11, the communicating the second user data comprising sending, by themobile device, a portion of the second user data to the CU via thesecond DU before participating in the handover.
 16. A mobile devicecomprising: a non-transitory memory storage comprising instructions; anda processor in communication with the memory storage, wherein theprocessor executes the instructions to: communicate first user data witha radio control protocol entity of a centralized unit (CU) via a firstlink layer entity of a first distributed unit (DU), the first link layerentity associated with the radio control protocol entity; receive anindication for the mobile device to transfer a link layer context to asecond distributed unit (DU); participate in a handover from the firstDU to the second DU; and communicate second user data with the radiocontrol protocol entity of the CU via a second link layer entity of thesecond DU, the second link layer entity associated with the radiocontrol protocol entity.
 17. The mobile device of claim 16, wherein theprocessor executing the instructions to communicate the second user datacomprises the processor executing the instructions to receive a portionof the second user data from the CU via the first DU interposed betweenthe CU and the second DU.
 18. The mobile device of claim 16, wherein theprocessor executes the instructions to receive Layer 2 configurationinformation related to a connection between the mobile device and thefirst DU.
 19. The mobile device of claim 16, wherein the processorexecuting the instructions to communicate the first user data comprisesthe processor executing the instructions to send a portion of the firstuser data to the CU via the first DU after participating in thehandover.
 20. The mobile device of claim 16, wherein the processorexecuting the instructions to communicate the second user data comprisesthe processor executing the instructions to send a portion of the seconduser data to the CU via the second DU before participating in thehandover.